1. Field of the Invention
The present invention relates to lithographic projection apparatus and more particularly to the determination of stray radiation condition therein.
2. Description of the Related Art
The term “patterning device” as here employed should be broadly interpreted as referring to means that can be used to endow an incoming radiation beam with a patterned cross-section, corresponding to a pattern that is to be created in a target portion of the substrate; the term “light valve” can also be used in this context. Generally, the said pattern will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit or other device (see below). Examples of such a patterning device include:                mask: the concept of a mask is well known in lithography, and it includes mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. Placement of such a mask in the radiation beam causes selective transmission (in the case of a transmissive mask) or reflection (in the case of a reflective mask) of the radiation impinging on the mask, according to the pattern on the mask. In the case of a mask, the support structure will generally be a mask table, which ensures that the mask can be held at a desired position in the incoming radiation beam, and that it can be moved relative to the beam if so desired;        programmable mirror array: an example of such a device is a matrix-addressable surface having a viscoelastic control layer and a reflective surface. The basic principle behind such an apparatus is that (for example) addressed areas of the reflective surface reflect incident light as diffracted light, whereas unaddressed areas reflect incident light as undiffracted light. Using an appropriate filter, the said undiffracted light can be filtered out of the reflected beam, leaving only the diffracted light behind; in this manner, the beam becomes patterned according to the addressing pattern of the matrix-addressable surface. The required matrix addressing can be performed using suitable electronic means. More information on such mirror arrays can be gleaned, for example, from U.S. Pat. Nos. 5,296,891 and 5,523,193, which are incorporated herein by reference. In the case of a programmable mirror array, the said support structure may be embodied as a frame or table, for example, which may be fixed or movable as required; and        programmable LCD array: an example of such a construction is given in U.S. Pat. No. 5,229,872, which is incorporated herein by reference. As above, the support structure in this case may be embodied as a frame or table, for example, which may be fixed or movable as required.        
For purposes of simplicity, the rest of this text may, at certain locations, specifically direct itself to examples involving a mask and mask table; however, the general principles discussed in such instances should be seen in the broader context of the patterning device as set forth above.
Lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the patterning device may generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising one or more dies) on a substrate (silicon wafer) that has been coated with a layer of radiation-sensitive material (resist). In general, a single wafer will contain a whole network of adjacent target portions that are successively irradiated via the projection system, one at a time. In current apparatus, employing patterning by a mask on a mask table, a distinction can be made between two different types of machine. In one type of lithographic projection apparatus, each target portion is irradiated by exposing the entire mask pattern onto the target portion in one go; such an apparatus is commonly referred to as a wafer stepper. In an alternative apparatus—commonly referred to as a step-and-scan apparatus—each target portion is irradiated by progressively scanning the mask pattern under the projection beam in a given reference direction (the “scanning” direction) while synchronously scanning the substrate table parallel or anti-parallel to this direction; since, in general, the projection system will have a magnification factor M (generally <1), the speed V at which the substrate table is scanned will be a factor M times that at which the mask table is scanned. More information with regard to lithographic devices as here described can be gleaned, for example, from U.S. Pat. No. 6,046,792, incorporated herein by reference.
In a manufacturing process using a lithographic projection apparatus, a pattern (e.g. in a mask) is imaged onto a substrate that is at least partially covered by a layer of radiation-sensitive material (resist). Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book “Microchip Fabrication: A Practical Guide to Semiconductor Processing”, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 007-067250-4, incorporated herein by reference.
For the sake of simplicity, the projection system may hereinafter be referred to as the “lens”; however, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, and catadioptric systems, for example. The radiation system may also include components operating according to any of these design types for directing, shaping or controlling the projection beam of radiation, and such components may also be referred to below, collectively or singularly, as a “lens”. Further, the lithographic apparatus may be of a type having two or more substrate tables (and/or two or more mask tables). In such “multiple stage” devices the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposures. Twin stage lithographic apparatus are described, for example, in U.S. Pat. No. 5,969,441 and WO 98/40791, incorporated herein by reference.
There is a need to produce smaller and smaller semiconductor devices, and thus a corresponding need to provide projection systems enabling projection of features with smaller critical dimension (“CD”). Thus, these lithography apparatus are being pushed to their resolution limits, while maintaining sufficient “process latitude” (i.e., sufficient depth of focus and sufficient insensitivity to residual errors in the dose of exposure of irradiated target portions). Therefore, there is a need to minimize factors which can affect the resolution of the apparatus and the process latitude, and consequently there is a need to provide accurate monitoring means to monitor these factors.
Many different factors can affect the smallest CD and the process latitude for a given lithographic projection apparatus, such as, for example, residual aberrations of the projection system, focus and dose errors, and the occurrence of stray radiation. In particular, the more stray radiation is present in the image of a pattern projected by the projection system, the less resolution can be obtained, and the smaller the process latitude will be. Stray radiation may, for example, be caused by scattering of projection beam radiation at contaminating particles and/or defects on surfaces of optical elements of the projection system. Also, optical elements provided with anti-reflection coatings may cause stray radiation due to degradation of materials used for the anti-reflection coatings. Degradation of materials may be a radiation induced effect, and like the number of contaminating particles and/or defects it may increase as a function of time. The word “condition” in the expression “a stray radiation condition,” as used above, refers to a momentary state of the presence of stray radiation causing effects in the projection system. The stray radiation condition is typically a time-dependent quantity. When the amount of stray radiation increases, the stray radiation condition deteriorates.
To more precisely assess the stray radiation condition of a projection system, it is useful to identify “ranges” of stray radiation. A point in the object plane and irradiated by radiation of the projection beam is traversed by rays of the projection beam. These rays subsequently traverse the projection system whereby a sub-set of the rays is diverted due to scattering. This sub-set of rays intersect a plane conjugated to the object plane (referred to hereinafter as “image plane”) in an area comprising the geometrical image of said point at a corresponding subset of distances from that geometrical image point. The distances can be ordered into ranges of distances, and these ranges constitute “ranges” of stray radiation. Generally, the projection beam of radiation provided by the radiation system is such that the rays traversing said point in the object plane are symmetrically distributed with respect to the optical axis of the projection apparatus.
However, despite this situation, the spatial intensity distribution of the stray radiation in the image plane may not be symmetric with respect to the geometrical image point. Thus, the range of stray radiation may be an asymmetric function of X and Y coordinates of a Cartesian coordinate system in the image plane with its origin 0 coincident with the geometric image point. This may be indicative for a certain class of defects in the projection system.
Measures for stray radiation in different ranges can be used in a simulator to estimate the degradation of CD, and hence to estimate the effect on the resolution performance of the lithographic projection apparatus. The resolution performance can become out of tolerance and therefore there is the problem of accurately monitoring stray radiation for different ranges, so that preventive measures such as cleaning lens surfaces can be taken in time. Further, although lithographic projection apparatus are generally provided with means to measure resolution performance, it is not possible from such measurements to assess in situ and sufficiently fast the presence of stray radiation in different ranges and to assess the necessity of taking appropriate measures to reduce stray radiation.